The N-terminal globular domain of Eph receptors is sufficient for ligand binding and receptor signaling

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Abstract
The Eph family of receptor protein‐tyrosine kinases (RTKs) have recently been implicated in patterning and wiring events in the developing nervous system. Eph receptors are unique among other RTKs in that they fall into two large subclasses that show distinct ligand specificities and for the fact that they themselves might function as ‘ligands’, thereby activating bidirectional signaling. To gain insight into the mechanisms of ligand–receptor interaction, we have mapped the ligand binding domain in Eph receptors. By using a series of deletion and domain substitution mutants, we now report that an N‐terminal globular domain of the Nuk/Cek5 receptor is the ligand binding domain of the transmembrane ligand Lerk2. Using focus formation assays, we show that the Cek5 globular domain is sufficient to confer Lerk2‐dependent transforming activity on the Cek9 orphan receptor. Extending our binding studies to other members of both subclasses of receptors, it became apparent that the same domain is used for binding of both transmembrane and glycosylphosphatidyl‐anchored ligands. Our studies have determined the first structural elements involved in ligand–receptor interaction and will allow more fine‐tuned genetic experiments to elucidate the mechanism of action of these important guidance molecules.